1. Field of the Invention
The present invention relates to a recording apparatus for recording an image on a recording medium on the basis of input data.
2. Related Background Art
As conventional non-impact type printers using an electrophotographic process, a laser printer using a laser diode, an LED printer using a light-emitting diode array as a light source, a liquid crystal printer using a liquid crystal shutter, and the like are known. Of these printers, some laser printers which can change a density of dots of an image to be output can change a dot density using a signal from an external apparatus or a select switch.
However, when a dot density is merely changed, an image is nonuniformly formed or reproducibility of a detailed portion is considerably degraded.
Printers such as a laser printer utilizing an electrophotographic technique process a binary image signal. When these printers output a halftone image, an image signal is subjected to processing such as dither processing by a host computer and is converted to a binary signal before it is input to a printer. The binary signal is input to the printer.
As a method of inputting multi-value image data to a laser printer or the like and outputting a halftone image, a method wherein an input image signal is converted to an analog image signal, and the analog image signal is compared with a cyclic analog pattern signal such as a triangular-wave signal, thus generating a pulse-width modulated binary image signal, is proposed (e.g., Japanese Patent Laid-Open (Kokai) No. 62-42693).
FIG. 22 is a block diagram showing an arrangement of the apparatus according to this method.
Multi-value image data sent from a host computer (not shown) is subjected to digital-to-digital conversion by a D/D converter 202 for density correction, and is then latched by a latch circuit 203 in response to an image clock signal CLK, thus synchronizing timings. The latched image data is converted to an analog signal VA by a D/A converter 204. The analog signal VA is compared by a comparator 205 with a triangular wave having a predetermined period, which is output from a triangular wave generating circuit 219. Thus, the analog signal is converted from a level signal to a time modulation signal, i.e., a pulse-width modulated signal. The pulse-width modulated output from the comparator 205 is input to a laser driver 206, thus driving a laser diode 207.
A laser beam emitted from the laser diode 207 is reflected by a rotating polygon mirror 208 and is converted to a scan beam. The scan beam is focused on a photosensitive body 210 by a lens 209 having f-.theta. characteristics and is converted to a constant-speed scan beam. Note that some scan beam components are received by a beam detection device (not shown) to generate a known beam detection (BD) signal as a horizontal sync signal for the printer. The BD signal is used as a sync signal for a signal generator.
The photosensitive body 210 is uniformly charged by a charger 211, and is then irradiated with the laser scan beam. Thus, an electrostatic latent image is formed on the surface of the photosensitive body, and is then developed by a developing unit 212. The developed pattern is transferred onto a transfer material 214 by a transfer charger 213, and is fixed thereon by thermal fixing rollers 215 and 216. A developing agent left on the surface of the photosensitive body 210 without being transferred is recovered by a cleaner 217. A residual charge on the photosensitive body 210 is erased by a pre-exposure beam. Thereafter, the same image formation process is repeated.
In the conventional apparatus, a resolution of a halftone image (to be referred to as a number of lines) is determined depending on a printer.
However, when an output image of a printer is used as a block copy of a printing apparatus, the number of lines may often be changed from 50 to 100 in light printing, 125 to 200 in high-quality printing, or according to a favor of a user.
When the host computer performs dither processing and the printer outputs an image, the host computer may change a matrix pattern of dither processing to change the number of lines. In this case, however, some printers have different correspondences between dither patterns and densities. Even if the same dither pattern is input, one printer may output a hard halftone image, and another printer outputs a soft halftone image. Even in a given printer, when the number of lines is changed, a halftone image density may vary.
As described above, a printer apparatus using an electrophotographic technique such as a laser beam printer has been widely used as an output apparatus of a computer. A printer apparatus of this type has many merits such as high image quality, low noise, and the like, and is increasingly used in the field of desk-top publishing (to be referred to as DTP hereinafter) in view of, especially, high image quality.
At the same time, along with development of a host computer or a controller for controlling the printing apparatus, various processing operations are available in halftone reproduction.
For example, when a halftone image is obtained as a hard copy, an area gradation method based on a dither method or the like is widely used at present.
In this method, a pixel density of a printer apparatus is set to be, e.g., 300 dpi, an 8.times.8 (pixel) matrix is formed, and pixels in the matrix are printed according to image data, thereby expressing gradation. In this case, the number of lines upon halftone reproduction is 37.5 lines per inch, and the number of gradation levels is 64. The number of lines and the number of gradation levels are not limited to the above numerical values but various other values may be employed according to necessity of an operator. For example, 75 lines and 16 gradation levels can be realized by a 4.times.4 (pixel) matrix.
The dither method is classified into a distributed type and a concentrated type depending on the way of filling pixels.
FIGS. 28A and 28B exemplify 4.times.4 (pixel) dither matrices and dot patterns. FIG. 28A shows a sample of the distributed type method, and FIG. 28B shows a sample of the concentrated type method. In FIGS. 28A and 28B, numerical values indicate the order of forming dots. The number of dots is increased in turn from low-density data on the basis of a predetermined threshold value.
In the conventional technique, however, only one dot pattern is prepared for one 8.times.8 (pixel) or 4.times.4 (pixel) matrix, and no countermeasure is taken for selecting a printing density of a printer engine. Thus, the following drawbacks are posed.
For example, in a printer in which a printer engine has two printing densities of 300 dpi and 600 dpi, and a 4.times.4 dither matrix is used, when the printing density is switched from 300 dpi to 600 dpi to express gradation while the 4.times.4 dither matrix remains the same, the number of lines is doubled, and a finer expression is allowed. However, at gradation level "1", i.e., when a dot is formed on one pixel in the matrix, since reproducibility per pixel is degraded due to the doubled number of lines, it is difficult to express this gradation level, and a completely blank portion is formed.
In contrast to this, at gradation level "15", i.e., when only one blank pixel is formed among black pixels, since its reproducibility is low, a resultant image is in solid black.
For this reason, the number of gradation levels to be reproduced is decreased, and an image having high black-and-white contrast, i.e., having a so-called steep gamma curve is undesirably obtained.
A laser beam printer is widely known in which a latent image is formed on a rotary drum by scanning a laser beam using a rotary polygon mirror or the like and is transferred onto a paper sheet after it is developed, thereby recording an image.
FIG. 39 shows an arrangement of a conventional laser beam printer, and a description will be made below with reference to FIG. 39.
In FIG. 39, a paper cassette 702 stores paper sheets 701 as recording media. Uppermost one of the paper sheets 701 in the cassette 702 is separated by a sheet feed cam 703, so that its leading edge portion is fed to sheet feed rollers 704 and 704'. The cam 703 is intermittently rotated every sheet feed operation. A reflection type photo-sensor 718 detects light reflected by the sheet 701 through a hole 719 formed in the bottom portion of the paper cassette 702 to detect the presence/absence of paper sheets.
When the paper sheet 701 is fed to a gap portion between the sheet feed rollers 704 and 704' by the sheet feed cam 703, the rollers 704 and 704' are rotated while lightly pressing the paper sheet 701 to feed the sheet 701. When the sheet 701 is fed and its leading edge reaches a position of a registration shutter 705, the paper sheet 701 is stopped by the registration shutter 705, and the sheet feed rollers 704 and 704' are kept rotated while slipping on the paper sheet 701 to generate a feed torque. In this case, when a registration solenoid 706 is driven to release the registration shutter 705 upward, the paper sheet 701 is fed to convey rollers 707 and 707'. The registration shutter 705 is driven in synchronism with a given timing of an image formed by focusing a laser beam 720 on a photosensitive drum 711. A photo-sensor 721 detects whether or not the paper sheet is present at the position of the registration shutter 705.
A rotary polygon mirror 75 is driven by a polygon mirror motor 753, and guides the beam 720 from a semiconductor laser 751 onto the photosensitive drum 711 via a reflection mirror 754, thereby forming a recording image on the photosensitive drum 711. A beam detector 755 arranged at a scan start position of the beam 720 detects the beam 720 to output a known BD signal which defines an image write timing in a main scan direction.
Thereafter, the paper sheet gains a feed torque by the convey rollers 707 and 707' in place of the sheet feed rollers 704 and 704', and is fed to the photosensitive drum 711 portion. An image exposed on the photosensitive drum 711 is transferred onto the paper sheet 701 in cooperation of a cleaner 712, a charger 713, a developing unit 714, and a transfer charger 715. The paper sheet 701 on which an image has been transferred is subjected to fixing processing by fixing rollers 708 and 708', and is then ejected onto a stacker 710 by sheet eject rollers 709 and 709'.
In FIG. 39, symbol A designates a guide for regulating a feed direction of the paper sheet 701.
A sheet feed table 716 allows to manually supply sheets one by one therefrom in addition to an automatic sheet feed operation from the paper cassette 702. A paper sheet manually fed to a gap portion between a manual sheet feed roller 717 and the sheet feed table 716 is fed while being lightly pressed by the roller 717 until its leading edge reaches the registration shutter 705. When the paper sheet has reached the shutter 705, the manual sheet feed roller slips. The following feed sequence is the same as that in the cassette sheet feed operation.
Note that the fixing roller 708 houses a fixing heater 724. The heater 724 controls a surface temperature of the fixing roller 708 to a predetermined temperature on the basis of a temperature detected by a thermistor 723 which is in slipping contact with the roller surface, thereby thermally fixing a recorded image on the paper sheet 701. A photo-sensor 722 detects whether or not a paper sheet is present at the position of the fixing rollers 708 and 708'.
The printer described above is not solely used but is connected to a controller through an interface cable. The printer receives a print command and an image signal from the controller to perform a print sequence. The arrangement of the interface cable and signals exchanged through the interface cable will be briefly described below.
FIG. 40 shows interface signals between a conventional printer and a controller.
As a printer 700, the laser beam printer described with reference to FIG. 39 can be employed.
Interface signals will be described below: PA1 PPRDY Signal . . . informs the controller that the power switch of the printer is turned on and is ready PA1 CPRDY Signal . . . informs the printer that the power switch of the controller is turned on PA1 RDY Signal . . . informs that the printer is ready to start or continue a print operation when it receives a PRNT signal (to be described later) from the controller PA1 PRNT Signal . . . an instruction signal from the controller to the printer, which causes the printer to start the print operation or to continue the print operation during the print operation of the printer PA1 VSREQ Signal . . . indicates that both the RDY and PRNT signals are at TRUE level and the printer is ready to receive a VSYNC signal (to be described later) PA1 VSYNC Signal . . . a vertical (sub-scan) sync signal of an image to be printed, which is output from the controller to the printer to synchronize an image on the drum with a paper sheet PA1 BD Signal . . . a horizontal (main scan) sync signal of an image to be printed, which indicates that a laser beam is present at a main scan start position PA1 VDO Signal . . . an image signal to be printed output from the controller. The printer outputs a black image in response to the TRUE-level VDO signal, and outputs a white image in response to the FALSE-level VDO signal PA1 SC Signal . . . a bidirectional serial 8-bit signal for exchanging an instruction signal COMMAND (to be described later) from the controller to the printer, and a status information signal STATUS from the printer to the controller. Both the controller and the printer use an SCLK signal (to be described later) as a sync signal when this signal is exchanged. Since this signal is a bidirectional signal, an SBSY signal and a CBSY signal (to be described later) are used for I/O control. PA1 SCLK Signal . . . a sync pulse signal used by the printer to fetch the signal COMMAND or by the controller to fetch the signal STATUS PA1 SBSY Signal . . . a signal for occupying an SC signal line and an SCLK signal line prior to transmission of the signal STATUS from the printer PA1 CBSY Signal . . . a signal for occupying the SC signal line and the SCLK signal line prior to transmission of the signal COMMAND from the controller PA1 GNRST Signal . . . a reset signal output from the controller to initialize the printer
For example, when paper sheets in the paper cassette 702 are used up and the print operation cannot be executed, the RDY signal goes to FALSE level.
The printer starts the print operation upon reception of this signal.
The signal COMMAND is an 8-bit serial signal, and includes various control commands to the printer, such as a sheet feed instruction for turning off only the fixing heater of the printer to keep an energy-saving state, i.e., to set a so-called sheet feed state, a sheet feed cancel instruction for canceling the sheet feed state and turning on the fixing heater, a cassette sheet feed instruction for supplying paper sheets from the paper cassette, a manual sheet feed instruction for manually supplying paper sheets, and the like.
The signal STATUS is an 8-bit serial signal, and informs the printer of various states, i.e., that the printer state is a wait state in that the temperature of a fixing device does not yet reach a print temperature, a paper jam occurs, there are no paper sheets in the paper cassette, and so on.
The operations between the printer and controller sections will be described below with reference to the system diagram showing connections between the printer and the controller.
Assume that the power switch of the printer is turned on, and the power switch of the controller is turned on. In this case, the printer initializes its internal states, and transmits the PPRDY signal to the controller. On the other hand, the controller initializes its internal states, and outputs the CPRDY signal to the printer. Thereafter, the printer energizes the fixing heater 724 housed in the fixing roller 708, and when the surface temperature of the fixing rollers 708 and 708' reaches a fixing temperature, the printer outputs the RDY signal to the controller.
Upon reception of the RDY signal, the controller transmits the PRNT signal according to necessity of the print operation. When the printer receives the PRNT signal, it rotates the photosensitive drum 711 to uniformly initialize the surface voltage of the photosensitive drum, and simultaneously drives the sheet feed cam 703 in a cassette sheet feed mode to feed the leading edge portion of a paper sheet to the position of the registration shutter 705. In a manual sheet feed mode, a paper sheet manually supplied from the sheet feed table 716 is fed to the position of the registration shutter 705. When the printer is ready to receive the VDO signal, it transmits the VSREQ signal to the controller.
Upon reception of the VSREQ signal, the controller transmits the VSYNC signal to the printer. When the printer receives the VSYNC signal, it drives the registration solenoid 706 in synchronism with the VSYNC signal to release the registration shutter 705. Thus, the paper sheet is fed to the photosensitive drum 711. After the controller outputs the VSYNC signal, it sequentially transmits the image signal VDO to be recorded to the printer in synchronism with the BD signal transmitted from the printer as a horizontal sync signal.
The printer turns on/off a laser beam in accordance with the VDO signal to form a latent image on the photosensitive drum 711. The latent image is developed with a toner in the developing unit 714, and the toner image is transferred onto the paper sheet by the transfer charger 715. Thereafter, the transferred image is fixed by the fixing rollers 708 and 708'. The paper sheet is then ejected.
When the sheet feed mode of the printer is switched to the cassette sheet feed mode or the manual sheet feed mode, the controller transmits an 8-bit serial code corresponding to the sheet feed mode to be selected to the printer in synchronism with the SCLK pulse signal through the SC signal line. When the printer receives the cassette sheet feed mode code, the printer is switched to a mode wherein the sheet feed roller 717 is not driven in the print operation, and the sheet feed cam 703 is driven to supply paper sheets from the cassette.
On the other hand, when the printer receives the manual sheet feed mode code, the printer is switched to a mode wherein the sheet feed cam 703 is not driven in the print operation, and the manual supply roller 717 is driven to allow a manual sheet feed operation.
When the power switch of the printer is initially turned on, the printer sets the sheet feed mode in the "cassette sheet feed mode" as an initial mode.
The GNRST signal is used to initialize the printer upon instruction from the controller. When the printer receives the GNRST signal from the controller, it resets all the executing jobs, and is reset to a state immediately after power-on. When a plurality of printers are connected to the controller, this signal is used to set the connected printers to an identical state.
In the conventional arrangement, a printer and a computer as a controller normally exchange interface signals through a connection cable having a length of 1 meter to several meters. A computer connected to the printer is not limited to one type, but various computers are often connected to the printer.
In this case, a conventional printer has a fixed dot density to be output. Therefore, a necessary printer must be prepared in accordance with an image processing speed or a required output dot density of a computer to be connected. For example, a plurality of printers having the fixed numbers of output dots in units of output dot densities like a 200-dpi printer having a dot density of 200 dots per inch, 240-dpi, 300-dpi, 400-dpi, and 480-dpi printers, and the like, and a model which can satisfy a requirement must be selected from these printers. In another improved printer, a dot density select switch is provided to an operation panel of the printer, and is manually switched by a serviceman or user to select a desired output dot density. In another printer free from a manual operation, the printer receives a dot density select command from the computer through a communication line, and rotations of a polygon scanner motor are changed in accordance with the designated dot density.
The printer which changes a dot density by the select switch on the operation panel or the printer which changes a dot density upon reception of a dot density change command from a computer suffers from the following problem.
In the conventional printer described above, when the printing density is selected upon an instruction from an external controller through a communication means such as a command, a scan speed in the main scan direction and that in the sub-scan direction are changed accordingly. In the conventional laser beam printer, an input image signal is masked so that unnecessary printing is not made on predetermined width portions of the leading, trailing, left, and right edge portions of a paper sheet to be subjected to printing (i.e., top, bottom, left, and right margins). An invalid image signal which may be transmitted from the controller, i.e., an image signal which is set at a timing falling outside a printing region is masked by an image signal masking means arranged in the printer, thus preventing an invalid printing operation on predetermined width regions from the leading, trailing, left, and right edges of the paper sheet. This operation is performed to a phenomenon in that a toner becomes attached to a region other than a paper region due to an offset feed operation of the paper sheet with respect to the photosensitive drum, and as a result, the toner which is not transferred onto the paper sheet contaminates the interior of the printer, and becomes attached to the subsequent printing image.
Such mask control of an image signal in the main scan direction is performed such that a predetermined number of clock pulses are counted after reception of the BD signal to determine an image mask timing in the main scan direction. For the mask control in the sub-scan direction, a predetermined number of BD signals are counted to determine the image mask timing in the sub-scan direction.
However, in the conventional control method, when the printing density is switched, the rotational speed (rotations) of a scanner motor is changed, and the scan speeds in the main scan and sub-scan directions are changed. Therefore, the image mask area cannot be accurately controlled. As a result, a printing image is contaminated with an excess toner, and maintenance or the like of the apparatus becomes very cumbersome.
When the printer receives a dot density select command for a second printing density during a print operation at a first printing density and starts to change the rotational speed of the polygon scanner motor, a printing image is undesirably distorted.
As improved recording apparatuses, U.S. Pat. No. 4,528,561, issued Jul. 9, 1985, U.S. patent application Ser. No. 350,649 filed on May 11, 1989, (refiled as U.S. patent application Ser. No. 563,859 on Aug. 7, 1990) Japanese Patent Laid-Open (Kokai) Nos. 62-162547, 62-162548, 62-162549, 62-162550, 62-162551, 62-163462, and 63-39268 have been proposed. However, further improvements are demanded.